The present invention relates to a multilayered photosensitive material for the preparation of a flexographic printing plate or, more particularly, to a multilayered photosensitive material for the preparation of a flexographic printing plate which is suitable for direct plate making by direct image writing with an infrared laser beam.
When a flexographic printing plate is prepared by utilizing the photolithographic technology, it is usual to employ a multilayered photosensitive material prepared by successively forming a layer of a photosensitive resinous composition and an oxygen-shielding layer on the surface of a substrate. Along with the progress of electronic devices in recent years, it is now possible also in the field of the printing technology to undertake a computerized general control of the overall process involving inputting and compilation of the printing materials, proof reading and plate making so that the direct plate-making method is highlighted in which a flexographic printing plate can be prepared directly from digital data of the printing materials.
The direct plate-making method, which is also called the CTP (computer-to-plate) method, is advantageous over the conventional plate-making method utilizing a negative transparency because, as a result of direct plate making from digital data, correction of the printing images can be conducted by correcting the computer-recorded digital data without necessitating re-preparation of a corrected negative transparency so that the time, labor and cost for the preparation of a completed printing plate can be greatly decreased.
While several variations are proposed for the direct plate-making method, a method is proposed as the most promising in which the patterned image is formed in a masking layer formed on a printing plate having a photosensitive resinous layer and the printing plate is prepared by utilizing the patterned image in the masking layer because this method has best adaptability as an extension of the conventional photolithographic method.
A most versatile procedure for practicing the above mentioned prior art for plate making is that the masking layer is patternwise removed by melting with irradiation of a laser beam. This procedure, however, has problems that, in the patternwise removal of the masking layer by melting, the photosensitive resinous layer is adversely influenced by the laser beam and that the polymerizability of the photosensitive resinous layer is decreased by the atmospheric oxygen acting as a polymerization inhibitor because the photosensitive resinous layer after removal of the masking layer by melting is protected only insufficiently.
The above-mentioned phenomena result in degradation of the quality of the flexographic printing plate because the shoulder portion of the patterned resinous layer of the printing plate is rounded due to inhibition of the polymerization reaction in the resinous layer by the atmospheric oxygen to increase the dot gain in printing.
As a countermeasure to this problem, Japanese Patent Kokai 62-11851 proposes a method in which the waxy compound in the photosensitive resinous layer is contained in a gradient distribution of the concentration so as to decrease the influence of the atmospheric oxygen in printing plate preparation. This method, however, is not practicable due to insufficient barrier effect of the layer against oxygen.
Alternatively, Japanese Patent Kokai 47-31705 and 53-69284 propose a method in which the photosensitive resinous layer is shielded against the atmospheric oxygen by providing a separate barrier layer on the surface thereof. This method, however, has a defect that, with the barrier layers thus far employed, the patterned resinous layer of the flexographic printing plate obtained by development has a poor cross sectional profile.
Further, according to a method disclosed in International Publication 94/3838, patterning of the resinous layer is conducted by using a photosensitive material having a multilayered structure comprising a substrate, a photosensitive flexographic layer, a shielding layer and an infrared-sensitive layer transmitting no chemical rays followed by removal of the masking layer by melting with an infrared laser beam. This method, however, is defective because second ultraviolet exposure cannot be undertaken or the exposure time must be extended in the presence of the atmospheric oxygen.